Class Computer

public abstract class Computer {

public abstract String getRAM();
public abstract String getHDD();
public abstract String getCPU();

@Override
public String toString(){
return "RAM= "+this.getRAM()+", HDD="+this.getHDD()+", CPU="+this.getCPU();
}
}

Class ComputerFactory

public class ComputerFactory {

public static Computer getComputer(String type, String ram, String hdd, String cpu)

{    if("PC".equalsIgnoreCase(type))
          return new PC(ram, hdd, cpu);
     else if("Server".equalsIgnoreCase(type)) return new Server(ram, hdd, cpu);

      return null;
}
}

Class PC

public class PC extends Computer {

private String ram;
private String hdd;
private String cpu;

public PC(String ram, String hdd, String cpu){
this.ram=ram;
this.hdd=hdd;
this.cpu=cpu;
}
@Override
public String getRAM() {
return this.ram;
}

@Override
public String getHDD() {
return this.hdd;
}

@Override
public String getCPU() {
return this.cpu;
}

}

Class Server

public class Server extends Computer {

private String ram;
private String hdd;
private String cpu;

public Server(String ram, String hdd, String cpu){
this.ram=ram;
this.hdd=hdd;
this.cpu=cpu;
}
@Override
public String getRAM() {
return this.ram;
}

@Override
public String getHDD() {
return this.hdd;
}

@Override
public String getCPU() {
return this.cpu;
}

}

Class SingletonComputerFactory

public class SingletonComputerFactory {
private static SingletonComputerFactory singletonFactory;

// SingletonExample prevents any other class from instantiating

private SingletonComputerFactory() {

}

// Providing Global point of access

public static SingletonComputerFactory getSingletonFactory() {

if (null == singletonFactory) {

singletonFactory = new SingletonComputerFactory();

}
return singletonFactory;
}

public Computer getComputer(String type, String ram, String hdd, String cpu)
{   if("PC".equalsIgnoreCase(type))
        return new PC(ram, hdd, cpu);
     else if("Server".equalsIgnoreCase(type))
       return new Server(ram, hdd, cpu);
     return null;
}
}

Class TestFactory (Using Singleton)

public class TestFactory {

public static void main(String[] args) {
SingletonComputerFactory fc = SingletonComputerFactory.getSingletonFactory();
Computer pc = fc.getComputer("pc","2 GB","500 GB","2.4 GHz");
Computer server = fc.getComputer("server","16 GB","1 TB","2.9 GHz");
System.out.println("Factory PC Config::"+pc);
System.out.println("Factory Server Config::"+server);
}

}

Run time output

Factory PC Config::RAM= 2 GB, HDD=500 GB, CPU=2.4 GHz
Factory Server Config::RAM= 16 GB, HDD=1 TB, CPU=2.9 GHz